A Head Gimbal Assembly ("HGA") is an assembly comprising a slider, a suspension assembly, and some type of wire connections or lead assembly which provides an electrical path to and from the slider. This is also sometimes referred to as a Head Suspension Assembly ("HSA") or as a hard file `head.` (The `gimbal` referred to in the name describes the gimballing action which a small part of the suspension, the flexure, provides to the slider.)
A slider can be a rectangular block of some durable material, with some rails or other raised details which creates an air bearing when the slider is moved at high relative velocity over a disk, and with some electromagnetic structure on or near the rear (`down-wind`) surface of the slider body which performs the writing and reading of magnetic information to and from the disk. The slider material can be a tough non-magnetic material such as the tungsten carbide used in most, if not all, inductive and magneto-restrictive ("MR") thin film sliders. (The magnetic element is created through the use of a thin film structure hence the name thin film.)
MR is an acronym which stands for magneto-restrictive, an effect exploited to read very faint magnetic records from disks (and tapes). The MR element changes resistance in response to the static magnetic field that it is immersed in. This resistance change can be measured by measuring the voltage change if a constant current source is supplied to the element or by measuring current changes if a constant voltage source is used.
Electro-Static Discharge ("ESD") is the rapid flow of static electricity to an uncharged body. This can be very damaging to fine (i.e. small cross-section) leads in various electronic components, including the MR element of an MR slider. As the storage density goes up, the MR element must be reduced in width and thickness, which makes it increasingly vulnerable to ESD damage. For example, on present MR products, the discharge of the amount of charge that can be carried by the average human body at a potential of 100 volts (VERY low for static charges) is more than enough to destroy the MR element.
The problem is that the very small cross-section magneto-restrictive element in hard file sliders is very susceptible to damaging effects from electrostatic discharge between its terminals (or pads). The MR head is extremely sensitive to ESD. Static potentials of 100 volts or more on a human body are able to totally destroy the MR element, and potentials of 10 volts or more can cause some level of damage called EOS (or electrical over stress) damage to the MR element, damage which can cause recording instabilities.
There are a number of alternative MR head and ESD protection schemes which have been disclosed or patented. One workable scheme involves using solder to bridge a gap between the MR pads on the slider. The solder would be put down at wafer level, and removed at actuator level, so ESD damage would be eliminated for most of the manufacturing process. A laser would be used to melt the solder to open the gap, and this would be done as late as possible in the manufacturing process, possibly just as the actuator is ready to be merged into the file. The drawback to this method is that solder is not a nice material to use in manufacturing, and the laser solder removal process may not ever be 100% reliable.
Other schemes involve connecting the MR leads together somewhere along the head gimbal assembly (HGA), only opening the leads when necessary for testing and for building, e.g., into an actuator. Unfortunately, these schemes all suffer from the drawback that the MR element is only protected for a limited portion of the manufacturing process, not for all of it. Thus, they do not sufficiently decrease the likelihood that an MR head will be damaged by ESD.
Damage from higher voltages is relatively easy to detect through visual inspection of the MR element or a check of the MR element resistance. But EOS damage is significantly harder to detect, sometimes only being detectable after the MR head has been built into a hard file and put through final test, a very expensive place to detect a manufacturing problem. Not only is EOS damage extremely difficult to detect by component level tests but it is also almost impossible to prevent through normal or even extraordinary ESD control measures. By far the best situation would be that all EOS or ESD damage would be prevented at all levels of assembly, and the present invention strives to do that.